Gareth is always happy to help train young witches although Wendy shouldn't expect to learn how to turn a village to glass or anything particularly powerful/destructive. Gareth would likely focus on teaching her healing magic and maybe a few "fun" spells.
I'd really like to see Gareth/Gabriel interact as well. There are so many different things we could do with them. I just had an idea that Gareth could target Gabriel and work him for information. Blackbeard would be too guarded and to dangerous to try and mess with but the First Mate would be a different story. Gareth could disguise himself as a pirate or a merman caught up in a net or just about anything else to get Gabriel's attention and then use him for information.